Understanding Eagle Software

I am trying to understand voltage regulators. I started drawing some circuits on iCircuit to model current flow and voltage drops. iCircuit is great because it allows me to visualize what is happening. Unfortunately there is no LM317, only LM7805. So I found Eagle software and started drawing the circuit, but it doesnt seem to be able to model current flow and voltage drops via a simulation.

Can that be done in Eagle software?

Otherwise can anyone suggest a software with which I could model the simulation using the LM317.


Eagle doesn't have any simulation feature like that.

My father uses OrCAD for simulations. He complained to me every time I talked to him for like a month about how awful the interface is when he was working on a project with it. I expect his complaints to continue next time he uses it. Getting the libraries for parts that it doesn't have by default can be a nightmare.

I don't simulate my designs - he's doing DC-DC switching converters, so that's getting into stuff that you might need simulations to design (you can often just crib from the datasheets...). Most hobbyist designs aren't complicated enough that the simulation is a particularly valuable tool.

What do you want to get out of your simulation?

Spice is the free sim. It comes in different flavours, I'm using LTSpice.

I do have LTSpice, Im not very good with it.

What I want is a simple way to see how voltage regulators work, to understand. For example, using iCircuit I can model this:

Screenshot 2016-06-07 18.34.45.png

and see that in a 12V source circuit, with a 10kO resistor, 1.2mA of current flows, easily verifiable using Ohms law. Additionally I can see the voltage drop over the R. I can also compare that with creating a voltage divider in order to drop different values over the two R, depending on their resistance. So with 2 equal R, the voltage drop is exactly half, from 12 to 6 and 6 to 0V. The current has dropped to 600uA because of the additional equal R.

In this screenshot:

Screenshot 2016-06-07 18.38.24.png

I can understand how R in parallel are different in that the first half of the circuit will have the largest current flow because its the path of ‘least resistance’. Interestingly enough, the voltage drop across both R is total, from 12V to 0V.

Now in studying Vregs like the LM7805, I can see that:

Screenshot 2016-06-07 18.41.39.png

the voltage drop across the Vreg is exactly down to 5V as it should be. I can also note that when powered by 5V, it is not enough to produce 5V because there is a Vdrop across the Vreg itself of about 1.6V if I recall correctly. Since Vin must be > Vout + 1.6V, if Vin is 5V, then Vout would be about 3.4V. So I should explore why the resulting voltage is 0V according to iCircuit instead of 3.4V. I remember seeing in the datasheet that the minimum Vin for LM7805 was 7V, so Im guessing its not outputting 3.4V just because LM7805 was designed to require at least 7V to output anything at all. Even at 6V or 7V the Vout is 0V.
At 8V I get 5V out and the current at least in the first half of the circuit is 1.35mA and the LED lights up.
At 12V I get 5V out & the current jumps to 2mA and the LED lights up. But I dont know what amperage goes out. I have no clue what EA is.

Unfortunately, iCircuit doesnt have an LM317, which I would like to understand because it combines the Vreg and the voltage divider concepts. So I did try in LTSpice but I dont know how to finish the circuit:

Screenshot 2016-06-07 19.06.56.png

I don't see why you find it so hard to understand. Look at the LTspice diagram. The 317 keeps on raising the voltage on C2 untill the voltage across R2 is 1.25volt. (the chip measures between OUT and ADJ).

If the voltage across R2 is constant, the voltage across R1 must be also constant. (no significant current is flowing in or out of the ADJ pin).

If the voltage across R2 and R1 is constant, the voltage across C2 is also constant.

Same with the 7805. Current from IN to OUT raises the voltage on the output cap until the voltage between OUT and GND is 5volt. Leo..

Thanks Wawa, you've been super helpful and patient. I know, I tend to make things more complicated than they have to be.

So as current flows into the lm317, the voltage on the Vout terminal starts increasing from 0V up until the point where the V across R2 reaches 1.25V.

I understand that much. I guess I just wanted to see it in action. Even then I think it wouldnt matter because I would have to understand how the inside of the lm317 works in order to understand how changing R2 causes lm317 to behave.

Im sure Ill get to the point where I can understand that a little further down the road.


Oh but to stay true to Q&A format...how would I see the LTSpice sketch at work in a similar way to how iCircuit works? What I mean is, how can I run a simulation where I feed 12V into the LM317 and change the R2 value around to see how the output voltage changes?

I managed to get the model working in LTSpice. I got these two samples:



So as I increase R2, voltage increases on both legs from 5.18 to 7.45 and from 3.93 to 6.25V.